Optical bistable device

ABSTRACT

A method of operating and apparatus for an optical bistable device is disclosed in which an active laser medium is placed between two mirrors within an optical resonator and bistable switching is achieved by operating the active resonator across the transition borderline between the stable and unstable resonator regions. Several different embodiments are disclosed.

BACKGROUND OF THE INVENTION

This invention relates to an optical bistable device (OBC) for inherentpotential applications such as optical switching, optical memory,bistable logic, differential amplification, optical transistor,discrimination, clipping, limiting, pulse shaping, and for performing anumber of optical digital data processing functions. More specifically,the present invention relates to an optical bistable device in which anactive laser medium is placed between two mirrors within an opticalresonator whereby bistable switching is provided by operating the activeresonator across the transition borderline between the stable-unstableresonator regions.

In conventional optical bistable devices (OBC), nonlinear Fabry-Perotresonators containing saturable absorbers or nonlinear refractive-indexmaterial are utilized. A schematic diagram of such a device is shown inFIG. 1(a) wherein a laser beam is used for power input. Typicalcharacteristics of such an optical bistable system are illustrated inFIG. 1(b).

This type of optical bistable device is referred to as "intrinsicsystem". U.S. Pat. No. 3,610,731, issued on Oct. 5, 1971 to H. Seideland U.S. Pat. No. 3,813,605, issued on May 28, 1974 to A. Szoeke, bothdisclose an intrinsic system containing saturable absorbers. NonlinearFabry-Perot resonators containing nonlinear refractive index materialswere demonstrated and described by Gibbs et al in 36 Phys. Rev. Lett.1135 (1976). The phenomenon appearing in one of such intrinsic systemsis described in "Laser Focus", April 1982, page 79, which isincorporated herein by reference.

Another type of optical bistable device involves a so-called hybridsystem. In contrast to the intrinsic systems mentioned above, themicroscopic nonlinearity in hybrids is synthesized using electro-opticfeedback. The hybrid optical bistable containing a medium with anonlinear refractive index and an electrical feedback loop was initiallysuggested and experimentally illustrated by Smith and Turner in 30 Appl.Phys. Lett. 280 (1977). An example of an electro-optic hybrid analogueof a dispersive optical bistable device is shown in FIG. 2. Theoperating principle of such systems is described in "Laser Focus", April1982, page 81, which is incorporated herein by reference.

As mentioned above, various types of optical bistable devices which haveso far been demonstrated in either hybrid or intrinsic systems requirenonlinear refraction as the microscopic nonlinearity. For intrinsicoptical bistable systems, an external laser is generally required foroperation because of the need for high intensity and/or interference,while hybrid systems which could be driven by a broad band sourcerequire electro-optic feedback circuits.

More specifically, in the nonlinear Fabry-Perot resonators containingsaturable absorbers, which can be referred to as absorptive OB,comparatively large changes in absorption are required to give rise toabsorptive bistability. Generally, it is difficult to produce absorptiveOB because the absorber must saturate to a low level of residualabsorption as described in "Laser Focus", April 1982, on page 81.

The Fabry-Perot resonator containing nonlinear refractive indexmaterial, which can be referred to as dispersive OB, requires nonlinearmedia showing a large intrinsic nonlinear index of refraction which isdifficult to find. In addition, those systems requiring multiple-beaminterference need relatively coherent light and hence will generallyrequire external lasers for their operation.

For the hybrid optical bistable system, since the microscopicnonlinearity required in the cavity is synthesized by usingelectro-optic feedback, the switching speed of the device is generallylimited. In addition, an external laser beam is required in most of thehybrid optical bistable systems.

Now, in the study of input vs. output power characteristics of aflashlamp-pumped Nd:GGG rod geometry laser, the present inventor hasobserved that the output laser power increases with increasing flashlampinput power. As the flashlamp input power exceeds a certain power level,the laser output power is reduced to an insignificant level and laseraction ceases if the input power is further increased. At that stage,the flashlamp input power is reduced and no laser action is observeduntil the input power decreases to a second power level which is smallerthan the certain power level. As the input power is further reducedbelow the second power level, laser action resumes. A hysteresis effectis reproducible.

This OB phenomenon has not been noticed so far in prior art laserresonators, which may be because of the fact that laser operation in theregion of stable-unstable configuration transition has never beencarefully studied.

The present discovery led to the development of a more general conceptfor active optical bistable laser devices, as described herein.

SUMMARY AND OBJECTS OF THE INVENTION

Accordingly, it is a primary object of the present invention to providean optical bistable device which overcomes the shortcomings of prior artoptical bistable devices as mentioned above.

Another object of the present invention is to provide a new class ofoptical bistable devices which requires neither a saturable absorber nornonlinear refractive-index material in the resonator.

Yet another object of the present invention is to provide an innovativetype of optical bistable device which requires no electro-opticfeedback.

Still another object of the present invention is to demonstrate opticalbistable effects with an incoherent, broad band source or even withnon-optical means.

Another object of the present invention is to provide a new type ofactive bistable optical device which is composed simply of a Fabry-Perotresonator and an active laser medium with an excitation source.

Briefly described, the present optical bistable device comprises anoptical resonator comprising a pair of mirrors and an active lasermedium exhibiting lens effects between the mirrors, and a light source,wherein laser-action-induced focusing or defocusing in the active lasermedium provides the feedback necessary for optical bistability andhysteresis effects even with the use of noncoherent light sources.

With these and other objects, advantages and features of the inventionthat must become hereinafter apparent, the nature of the invention mustbe more clearly understood by reference to the following detaileddescription of the invention, the appended claims and to the severaldrawings attached herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a schematic illustration of a prior art optical bistablesystem of the intrinsic type;

FIG. 1(b) is a graph illustrating typical characteristics of the opticalbistable system of FIG. 1(a);

FIG. 2 is a schematic illustration of a prior art optical bistablesystem of the hybrid type;

FIG. 3 is a graph showing input vs output power characteristics of aflashlamp-pumped Nd:GGG rod geometry laser;

FIG. 4 is a schematic elevational view of one embodiment of the presentinvention being of a thin rectangular slab shape;

FIG. 5 is a schematic elevational view of another embodiment of thepresent invention;

FIG. 6 is a schematic elevational view of still other embodiments of thepresent invention;

FIGS. 7(a) and 7(b) are schematic illustrations of still otherembodiments of the present invention;

FIG. 8 is a graph illustrating a relationship between the input powerand the effective focal length and bistable switching or opticalbistability of the present invention under the conditions where bothf(P_(in)) and f(P_(ls)) are positive and wherein f(P_(in)) and f(P_(ls))are the focal length due to pump and laser induced focusing of theresonator;

FIG. 9 is a graph similar to FIG. 8 for the situation where f(P_(in)) ispositive and f(P_(ls)) is negative;

FIG. 10 is a graph similar to FIG. 8 for the situation where bothf(P_(in)) and f(P_(ls)) are negative; and

FIG. 11 is a graph similar to FIG. 8 for the situation where f(P_(in))is negative and f(P_(ls)) is positive.

FIG. 12 is a graph illustrating stable and unstable cavityconfigurations for the resonator of an optical bistable device inaccordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As mentioned previously, in the study of input vs output powercharacteristics of a flashlamp-pumped Nd:GGG rod geometry laser, thepresent inventor has observed that the output laser power increases withincreasing flashlamp input power. As the flashlamp input power exceededa certain level denoted by P_(in) (down) as shown in FIG. 3, the laseroutput power was reduced to an insignificant level and laser actionceased if the input power was further increased. At that stage, theflashlamp input power was reduced and no laser action was observed untilthe input power decreased to a level denoted by P_(in) (up) which wassmaller than P_(in) (down). As the input power was further reduced belowP_(in) (up), laser action resumed. A hysteresis effect was clear andreproducible. That discovery has led to the present invention, someembodiments of which are described hereinafter.

Surprisingly, the active optical bistable device of the presentinvention does not need coherent light, for it exhibits optical bistablecharacteristics in response to even incoherent input light. In addition,the laser medium of the present active optical bistable device per seexhibits the optical bistable characteristics, so that the presentdevice does not need any electro-optic feedback.

The laser medium may be a gas (e.g. CO₂, N₂, H₃, etc.), a liquid (e.g.organic dye) or a solid (e.g. activated laser crystals such as YAG, GGG,Ruby, GSGG, etc.), or semiconducting materials (such as GaAs, etc.).

Referring now in detail to the drawings, wherein like parts aredesignated by like reference numerals throughout, there is illustratedin FIG. 4 a homogeneous active medium 10 in thin rectangular slab withpump sources 11 and 12 on the opposite sides thereof for excitation ofthe active medium 10. In one embodiment, the medium may compriseneodymium doped YAG (yttrium aluminum garnet), GGG (gadolinium galliumgarnet), or GSGG (gadolinium scandium gallium garnet), or otheractivated laser crystals, while the pump sources 11 and 12 may compriseflashlamps or an LED (light emitting diode) array.

Two optically plane parallel end faces 13 and 14 at each longitudinalend of slab 10 of active medium are coated with antireflection (AR) thinfilm at the lasing wavelength of the active medium 10. Mirrors 15 and 16form an optical cavity which provides feedback for laser action in themedium 10. The surface toward the resonator cavity of mirror 16 has ahigh reflectivity (HR) coating while that of mirror 15 has a partiallytransmissive coating. The radii of curvature of mirrors 15 and 16 areproperly selected depending upon the operating condition of the bistableswitching. Extending parallel to the longitudinal axis 17 of the bodyare two planes 18 and 19 to produce a channel for excitation of theactive medium 10. The width of the active medium 10 is much larger thanits thickness.

The thickness of the active medium 10 is properly chosen such that theresonator can only allow the lowest order (spatial) mode to oscillate.

In order to explain the basic principles of the invention, the theorynecessary to analyze resonators that contain optical elements other thanthe end mirrors will first be explained. That theory will then beapplied to the case of a resonator containing a laser medium. Underexcitation, the laser medium acts like a lens of an effective focallength, f_(eff), which may be either positive or negative depending uponthe laser medium and excitation processes.

The pertinent parameters of a resonator equivalent to one with aninternal thin lens are:

    g.sub.1 =1-L.sub.2 /f-L.sub.0 /R.sub.1 g.sub.2 =1-L.sub.1 f-L.sub.0 /R.sub.2                                                  (1)

where L₀ =L₁ +L2-(L₁ L₂ /f) and f is the focal length of the internallens; L₁ and L₂ are the spacings between mirrors M₁, M₂ and the lens;and R₁ and R₂ are the radii of curvature of mirrors M₁, M₂.

The stability condition of the active resonator can be then expressedby:

    0<g.sub.1 g.sub.2 <1                                       (2)

All cavity configurations are unstable unless they correspond to pointslying in the area, as illustrated by FIG. 12, enclosed by a branch ofthe hyperbola g₁ g₂ =1 and the coordinate axes g₁ g₂ =0. The research inthe prior art is substantially directed to the stable region.

The general theory of optical bistability of this invention is asfollows. There is a laser medium whose effective focal length, f_(eff).depends upon both the pump (excitation) input power, P_(in), and thelaser power in the resonator P_(ls). It is presumed that the conditionin the precise function form of f_(eff) against P_(in) and P_(ls) iswritten in its most general form:

    1/f.sub.eff =1/f(P.sub.in)+1/f(P.sub.ls);                  (3)

where f(P_(in)) and f(P_(ls)) are the focal lengths due to pump- andlaser-induced focusing (or defocusing) of the resonator, respectively.Note that both f(P_(in)) and f(P_(ls)) may be either positive ornegative.

The pump-induced lens effect may arise from nonuniform gain distributiondue to nonuniform pumping, nonuniform temperature distribution andthermally-induced distortion. On the other hand, the laser-induced lenseffect may be a result of a self-focusing (or self-defocusing),transverse spatial hole burning, heating of laser medium due to anabsorption or laser energy, and optical distortion of the resonatormirrors due to local heating by laser radiation.

Equation (3) is the key expression for optical bistability in the novelfeatures of this invention. The effective focal length of the lasermedium can presumably be expressed by equation (3) due to pump- andlaser-action-induced lensing effects as mentioned above. As a result,the cavity g parameters of the active resonator as shown in equation (1)depend upon both the pump power and the laser power. In other words,under the same pumping conditions, the cavity g parameters with laseraction taking place have different values from those without laseroscillation in action.

The cavity g parameters have subsequent influence on the laser operationof the active resonator as shown in equation (2). The active resonatorcan generate coherent light output if the stability condition of the gparameters as shown in equation (2) is met. Under the circumstances wheneither g₁ g₂ >1 or g₁ g₂ <0 is satisfied, the cavity configurationbecomes unstable and laser action ceases. The condition specified by g₁g₂ =0 or g₁ g₂ =1 sets the borderline between the regions of stable andunstable configurations of the resonator.

As the operating condition of the active resonator, which changes withboth input power and laser power, crosses the borderline, the activeresonator undergoes a phase transition, i.e. stable-unstableconfiguration transition or lasing-nonlasing state transition, whereoptical switching occurs. Note that the functions of the pump sourcesare two-fold. First, they produce population inversion (i.e. gain) inthe laser medium, and secondly, they create a lensing effect in thelaser medium, as shown by the first term on the right hand side ofequation (3). In addition, the pump sources may take several differentforms. For example, they may be in the form of a noncoherent light, e.g.a flash-lamp in a solid-state (YAG, ruby, or GGG) laser or in the formof an electrical current, e.g. the injection current in a semiconductorlaser.

The conditions for bistability (hysteretic behavior) of this inventiondepend upon the signs (positive or negative) of f(P_(in)) and f(P_(ls)).Using a graphing method (see FIG. 8), the value of 1/f_(eff) is plottedagainst P_(in) with laser action as a parameter. The borderlinecondition for stable-unstable resonator transition (i.e., g₁ g₂ =1 or g₁g₂ =0) can be obtained from Eq. (1), which is indicated by the dashedline in FIG. 8. Note that both f(P_(in)) and f(P_(ls)) are positive inthis case.

The regenerative action underlying the switching is as follows.Increasing the pump input power, P_(in), increases the 1/f_(eff) valuewhich is equal to 1/f(P_(in)). Since the resonator is unstable, there isno laser action for P_(in) below point A. At point C, the resonatorcondition changes from a stable to an unstable state and laser actionceases, which is indicated by the transition from point C to point D.The bistable switching is clearly seen.

FIG. 9 shows bistable switching under the conditions where f(P_(in)) ispositive and f(P_(ls)) is negative. FIG. 10 describes the hystereticbehavior for the situation where both f(P_(in)) is negative andf(P_(ls)) is positive.

In order to clearly explain the present invention over the prior artintrinsic optical bistable system, some features are compared asfollows.

The optical resonator used in the intrinsic (passive) OB system isdesigned to provide a cavity for interference to take place such thatthe nonlinear Fabry-Perot interferometer can produce a transmissionspectrum for the external laser beam where maximum transmission occursunder the condition that the external laser beam is on-resonance withthe passive cavity while minimum transmission takes place under thecondition that the external laser beam is off-resonance with the passivecavity, whereas the optical resonator used in the present invention isdesigned to provide an optical feedback in the active cavity for laseraction to occur.

In the prior art laser resonators, most of the efforts which have beenmade so far are directed to maintain the cavity g parameters at the samevalues throughout the operating region of the laser in order to retainconstant laser beam parameters, e.g., spatial modes and beam divergence.Whereas, in the present invention, the operation of the laser resonatoris designed in such a way that its cavity g parameters change with theoperating conditions and deliberately make the g parameters cross theborderline for the stable-unstable configuration of the resonator as theoperating condition varies to achieve the ON-to-OFF and OFF-to-ONtransitions of laser action.

Additional embodiments of the present optical bistable devices aredescribed as follows, showing modifications of the present invention.

The features of the embodiment shown in FIG. 5 are similar to thosedescribed in FIG. 4 except that the cavity mirrors are not used and theend faces 23 and 24 of the active medium 10 are constructed as cavitymirrors for laser oscillation. The end face 24 has a high reflectivity(HR) coating on it whereas the end face 23 is coated with partiallytransmissive film for output.

Referring now to FIG. 6, another embodiment of the present inventioncomprises a rectangular semiconducting crystal (e.g. GaAs). The activelaser region 30 is in the p-n junction between the p-type region 31 andthe n-type region 32. Two plane faces 33 and 34 are metallized formaking contact with bias leads 35 and 36 through which the injectioncurrent 37 provides gain for the active medium in the p-n juction region30. Two optical end faces 38 and 39 form a resonant cavity for laseroscillation with the p-n junction region 30. The radius of curvature offace 39 is properly selected so that the laser resonant cavity definedthereby is unstable when no current is injected. The end face 39 iscoated with high reflectivity thin film at the laser wavelength. The endface 38 is either cleaved or polished for laser output.

The laser described herein provides a bistable output controlled by theinjection current. At low injection current, the resonant cavity isunstable. Therefore, no laser action is taking place. As the injectioncurrent reaches a certain level, the resonant cavity becomes stable dueto pump-induced lensing in the active region and laser action occurs.That process completes the switching ON of the device. At that stage, ifthe injection current is reduced, the resonant cavity will becomeunstable at a lower injection current when laser action ceases. Suchprocess concludes the OFF switching.

FIG. 7(a) shows still another alternate embodiment of the presentinvention. A homogeneous active medium 40 which exhibits very weak oreven no laser-induced lens effect is placed in an optical cavity formedby mirrors 41 and 42. A passive medium 43 showing strong laser-power (orintensity)-dependent lens effect is disposed in the cavity defined bymirrors 41 and 42. Two focusing lenses 44 and 45 are shown in FIG. (7a)are placed with the passive medium 43 sandwiched therebetween toincrease laser power density in the passive medium 43 such thatlaser-induced lens effect in the passive medium 43 is enhanced.

In FIG. 7(b), which also shows another embodiment of the presentinvention similar in manner to that shown in FIG. 7(a), the lensingenhancement described above is achieved by the curved surfaces 41 and46.

In the embodiments shown in FIGS. 7(a) and 7(b), the pump-induced lenseffect required for bistable operation is provided by the active medium40, while the laser-induced lens effect is produced in the passivemedium 43.

The active medium may be made of a gas (e.g., CO₂, He and N₂ and theirmixtures), liquid (e.g. dye solution) or solid (e.g. Ti³⁺ - and Cr³⁺-doped fluoride crystals). The passive medium 43 may likewise be made ofa gas (e.g. CS₂, liquid (e.g. Nitrobenzene), or solid (e.g. LiNbO₃).

While preferred embodiments of this invention have been shown anddescribed, it will be appreciated that other embodiments will becomeapparent to those skilled in the art upon reading this disclosure, and,therefore, the invention is not to be limited by the disclosedembodiments.

What is claimed is:
 1. An optical bistable device comprising:(a) anoptical resonator capable of resonating a laser beam therein when saidresonator has a stable geometry and incapable of resonating the sametherein when said resonator has an unstable geometry, wherein saidresonator includes means for changing from a stable geometry to anunstable geometry according to a change in intensity of said laser beamand magnitude of input energy supplied to said optical resonator, (b) apair of mirrors, (c) an internal refractive medium disposed between saidoptical resonator and pair of mirrors, (d) a light emitting substanceplaced in said optical resonator for emitting simulated light uponreceiving a supply of the input energy wherein said simulated lightforms a laser beam when resonated by said optical resonator, and theintensity of said laser beam depends upon the magnitude of the inputenergy; (e) means for suppying said resonator and said light emittingsubstance with the input energy; and (d) means for increasing anddecreasing the magnitude of the input energy over first and secondprescribed values, wherein the first prescribed value of the inputenergy is a value at which the resonator changes from a stable geometryto an unstable geometry when sufficient input energy exists for laseraction to occur under the stable geometry, and the second prescribedvalue of the input energy is a value at which the resonator changes froman unstable geometry to a stable geometry under the same conditionmentioned above.
 2. The optical bistable device of claim 1 wherein atleast one of said mirrors is a high reflectivity thin film coated on theend face of the internal refractive medium along an optical axis.
 3. Anoptical bistable device of claim 2 wherein said internal refractivemedium comprises a semiconductor material, and said light emittingsubstance comprises electrons included in the semiconductor materialwherein the electrons are capable of making the transition between aconduction band and a valence band whose energy gap corresponds to thefrequency of the laser beam.
 4. The optical bistable device of claim 1wherein said resonator includes means for changing from an unstablegeometry to a stable geometry according to a change in the magnitude ofinput energy supplied to said optical resonator.
 5. A method forproviding optical bistability using a laser system having an opticalresonator including a pair of mirrors and an internal refractive mediumdisposed therebetween wherein said optical resonator is capable ofresonating a laser beam therein when said resonator has a stablegeometry and incapable of resonating the same therein when saidresonator has an unstable geometry, and wherein said resonator iscapable of changing from a stable geometry to an unstable geometryaccording to a change in the intensity of said laser beam and magnitudeof input energy supplied to said optical resonator, said methodcomprising the steps of:(a) supplying the input energy to the resonatorhaving a stable geometry in order to excite the laser beam, wherein theintensity of the laser beam depends upon the input energy; (b)increasing the input energy so that the resonator changes from thestable geometry to the unstable geometry according to the change in theintensity of said laser beam and magnitude of input energy, resulting inthe cessation of laser action at a first prescribed value of the inputenergy; (c) decreasing the input energy so that the resonator changesfrom the unstable geometry to the stable geometry according to thechange in the magnitude of input energy, resulting in the resumption oflaser action at a second prescribed value of the input energy; wherebyan output beam of the laser system exhibits hysteresis when the inputenergy is increased and decreased over the first and second prescribedvalues.
 6. A method for providing optical bistability using a lasersystem having an optical resonator including a pair of mirrors and aninternal refractive medium therebetween wherein said optical resonatoris capable of resonating a laser beam therein when said resonator has astable geometry and incapable of resonating the same therein when saidresonator has an unstable geometry and wherein said resonator is capableof changing from a stable geometry to an unstable geometry according toa change in the intensity of said laser beam and magnitude of inputenergy supplied to said optical resonator, said method comprising thesteps of:(a) supplying the input energy into said resonator having anunstable geometry, (b) increasing the input energy so that saidresonator changes from the unstable geometry to stable geometryaccording to the change in the magnitude of input energy, resulting inthe initiation of laser action at a second prescribed value of the inputenergy, and (c) decreasing the input energy so that the resonatorchanges from a stable geometry to an unstable geometry according to achange in the intensity of the laser beam and magnitude of input energy,resulting in the cessation of laser action at a first prescribed valueof the input energy; whereby an output beam of the laser system exhibitshysteresis when the input energy is increased and decreased over thefirst and second prescribed values.
 7. An optical bistable devicecomprising:(a) an optical resonator comprising, a pair of mirrors facingand separated from each other, and an internal refractive mediumdisposed between the mirrors wherein said internal refractive medium iscapable of having a finite focal length changeable according to a changein an input power Pin and a laser power Pls, owing to the fact that thefocal length varies according to the change in the amount of heatgenerated in the refractive medium due to absorption of the input powerPin and laser power Pls, and wherein cavity parameters of the opticalcavity g₁ and g₂ are given by g₁ =(1-L₁ /f-L₀ /R) and g₂ =(1-L₂ /f-L₀/R) where L₀ =L₁ +L₂ -L₁ L₂ /f, L₁ and L₂ are effective spacing betweenthe internal refractive medium and mirrors, and R₁ and R₂ are radii ofcurvature of the mirrors, so that when Pin and Pls take valuescorresponding to 0<g₁ g.sub. 2 <1, a cavity configuration is opticallystable and laser action is capable of taking place, and when Pin and Plstake values corresponding to g₁ g₂ <0 or g₁ g₂ >1 the cavityconfiguration is optically unstable and laser action is incapable oftaking place; (b) a light emitting substance placed in said opticalcavity for emitting stimulated light upon receiving said supplies ofinput power wherein said stimulated light forms laser light whenresonated by said optical resonator and the laser power Pls depends uponthe input power Pin; (c) input means for inputting the input power intosaid internal refractive medium and said light emitting substance; (d)means for increasing and decreasing the input power Pin over first andsecond prescribed values wherein the first prescribed value of the inputenergy is a value at which the cavity parameters g₁ and g₂ changes froma value satisfying the relation 0<g₁ g₂ <1 to a value satisfying therelation g₁ g₂ <0 or g₁ g₂ >1 through change of the focal length fdepending upon Pin or Pls under the condition that a sufficient inputenergy is input such that a laser action occurs when the parameter g₁and g₂ satisfy the relation 0<g₁ g₂ <1, and the second prescribed valueof the input energy is a value at which the cavity parameter g₁ and g₂changes from a value satisfying the relation g₁ g₂ <0 or g₁ g₂ >1 to avalue satisfying the relation 0<g₁ g₂ <1 through change of the focallength f depending upon Pin, whereby an output beam of the opticalresonator exhibits hysteresis when the magnitude of said input power Pinis increased and decreased over first and second prescribed values. 8.The optical bistable device of claim 7, wherein the change in the focallength of the internal refractive medium is due to the distortion of theshape of the medium.
 9. The optical bistable device of claim 8, whereinsaid internal refractive medium comprises a solid medium whoserefractive index is different from unity, said light emitting substancecomprises at least one of the atoms and ions having energy levels whoseseparations of energy correspond to the frequency of said laser beam,and said light emitting substance is embedded in the solid medium. 10.The optical bistable device of claim 9, wherein said internal refractivemedium is a member selected from the group of crystals consisting ofYAG, GGG, Ruby, and GSGG.
 11. The optical bistable device of claim 7,wherein said mirrors comprise a Fabry-Perot resonator.
 12. The opticalbistable device of claim 7 wherein said internal refractive mediumcomprises a non-lasing solid medium whose focal length will depend uponthe input power Pin and the laser power Pls.
 13. The optical bistabledevice of claim 12, further comprising a focusing lens placed withinsaid non-lasing solid medium between said focusing lens and one of saidmirrors to increase laser power density in said non-lasing solid medium.14. The optical bistable device of claim 12, further comprising a pairof focusing lenses placed within said non-lasing solid mediumtherebetween to increase laser power density in said non-lasing solidmedium.
 15. An optical bistable device of claim 7, wherein an inputmeans for inputting the input energy comprises a light source whichemits an incoherent light.
 16. An optical bistable device of claim 7where said internal refractive medium comprises a liquid medium.
 17. Amethod for providing optical bistability using a laser system having aninternal refractive medium within an optical cavity defined by mirrorsof an optical resonator wherein said internal refractive medium has afocal length f changeable according to a change in an input power Pininput into the cavity from pumping means and a laser power Pls excitedin the cavity, owing to the fact that the focal length varies accordingto the change in the amount of heat generated in the refractive mediumdue to absorption of the electromagnetic energy and wherein cavityparameters of the optical cavity g₁ and g₂ are given by g₁ =(1-L₁ /f=L₀/R) and g₂ (1-L₂ /f-L₀ /R) where L₀ =L₁ +L₂ -L₁ L₂ /f, L₁ and L₂ areeffective spacing between the internal refractive medium and mirrors,and R₁ and R₂ are radii of curvature of the mirrors, so that when Pinand Pls take values corresponding to 0< g₁ g₂ <1, a cavity configurationis optically stable and laser action is capable of taking place, andwhen Pin and Pls take values corresponding to g₁ g₂ <0 or g₁ g₂ >1 thecavity configuration is optically unstable and laser action is incapableof taking place, said method comprising the steps of:(a) inputting theinput power into the optical resonator to excite a laser light in thecavity wherein laser power Pls depends upon the input power Pin, and thecavity parameter g₁ and g₂ satisfying the relation 0<g₁ g₂ <1; (b)increasing the input power Pin so that the cavity parameter g₁ and g₂change according to the change in the input power Pin through change ofthe focal length of the internal refractive medium f depending upon Pinand Pls, resulting in that the cavity parameters g₁ and g₂ changes froma value satisfying the relation 0<g₁ g₂ <1 to a value satisfying therelations g₁ g₂ <0 or g₁ g₂ >1, at first prescribed value of the inputpower Pin, so that the laser action is ceased at the same value of Pin;and (c) decreasing the input power Pin so that the cavity parameter g₁and g₂ change according to the change in the input power Pin throughchange of the focal length of the internal refractive medium dependingupon Pin, resulting in that the cavity parameters g₁ and g₂ change froma value satisfying the relations g₁ g₂ <0 or g₁ g₂ >1 to a valuesatisfying the relation 0<g₁ g₂ <1, at a second prescribed value of theinput power, so that the laser action is resumed at the same value ofPin, whereby an output beam of the optical resonator exhibits hysteresiswhen the magnitude of said input power Pin is increased and decreasedover the first and second prescribed values.
 18. A method for providingoptical bistability using a laser system having an internal refractivemedium within an optical cavity defined by mirrors of an opticalresonator wherein said internal refractive medium has a focal length fchangeable according to the change in an input power Pin input into thecavity from pumping means and a laser power Pls excited in the cavity,owing to the fact that the focal length varies according to the changein the amount of the heat generated in the refractive medium due toabsorption of the electromagnetic energy, and wherein cavity parametersof the optical cavity g₁ and g₂ are given by g₁ =(1-L₁ /f-L₀ /R) and g₂(1-L₂ /f-L₀ /R) where L₀ =L₁ +L₂ -L₁ L₂ /f, L₁ and L₂ are effectivespacing between the internal refractive medium and mirrors, and R₁ andR₂ are radii of curvature of the mirrors, so that when Pin and Pls takevalues corresponding to 0<g₁ g₂ <1, a cavity confiruration is opticallystable and laser action is capable of taking place, and when Pin and Plstake values corresponding to g₁ g₂ <0 or g₁ g₂ >1 the cavityconfiguration is optically unstable and laser action is incapable oftaking place, said method comprising the steps of:(a) inputting thepower into an optical resonator, wherein the cavity parameters g₁ and g₂satisfy the relation g₁ g₂ <0 or g₁ g₂ <1, so that laser action does nottake place; (b) increasing the input power Pin so that the cavityparameters g₁ and g₂ change according to a change in input power Pinthrough a change of the focal length of the internal refractive medium fdepending upon Pin, resulting in that the cavity parameter changes froma value satisfying the relation g₁ g₂ <0 or g₁ g₂ >1 to a valuesatisfying the relation 0<g₁ g₂ <1, at a second prescribed value of Pin,so that laser action is commenced at the same value of Pin; and (c)decreasing the input power Pin so that the cavity parameters g₁ and g₂change according to the change in the input power Pin through a changeof the focal length of the internal refractive medium f depending uponPin and Pls resulting in that the cavity parameter g₁ and g₂ change froma value satisfying the relation 0<g₁ g₂ <1 to a value satisfying therelation g₁ g₂ <0 or g₁ g₂ >1, at a first prescribed value of the inputpower Pin, so that the laser action is ceased at the same value of Pinwhereby an output beam of the optical resonator exhibits hysteresis whenthe magnitude of said input power Pin is increased and decreased overthe first and second prescribed values.